Nitrided metals



' stantial absence of air.

i trite This invention relates to certain nitrided metals and to a method for preparing same. In one aspect the invention relates to a method for preparing by powder metalurgy a nitrided metal shape or article having increased hardness throughout the body of the article. Other aspects of the invention will become apparent from a consideration of this specification.

The present invention is applicable to the group IV metals selected from titanium, zirconium and hafnium. These metals, notably titanium, have been previously nitrided by prior art methods by contacting a titanium metal shape or object with ammonia at temperatures which cause the decomposition of the ammonia. Such treatments result in nitriding only a very thin skin of the surface, but not the bulk of the articles, in order to impart a surface hardness. It has also been found that these methods result in severe embrittlement because of the presence of hydrogen in the interstices of the metal; this embrittlement is especially severe when hydrogen is allowed to diffuse while the metal is in the beta crystal form which is stable above 1620 F. This hydrogen embri-ttlement of titanium is even more severe when the metal specimen is slowly cooled through the temperature range of about 392 to 572 F., resulting in the precipitation of large agglomerates of the hydride which appear as line markings in photomicrographs taken of thin sections of the metal. With quick quenching through this temperature range the problem is somewhat alleviated since the hydride is precipitated in a more highly dispersed condition and thus causes a less drastic weakeniug along the crystal lattice planes. Even with this pre-' caution the metal is highly brittle and of little use where the part is to be subjected to high energy impacts.

It is, therefore, an object of the invention to prepare hard nitrided metal objects which have an increased hardness throughout a cross section of the object. It is also an object of the invention to prepare nitrided metal objects or shaped articles while avoiding embrittlernent thereof because of the presence of hydrogen. It is another object of the invention to prepare nitrided metal objects of increasedhardness throughout and to then further nitride the surface of such object to attain an even harder surface than the body of the object.

Other objects, as well as advantages, of the invention,

will become apparent from the accompanying disclosure.

According to one aspect of the invention there is provided a process wherein a powdered metal selected from the group consisting of titanium, zirconium and hafnium is intimately admixed with a powder of the selected metal which has been nitrided and placed in a mold, which is preferably evacuated, the mixture of powders is subjected to a molding pressure, usually in the range from 50 to 100 tons per square inch, and thereafter the resulting molded article is heated at a temperature in the range from 1400 to 3000" F., usually from 2000 to 3000 F., in the sub- This heating step is usually done in an evacuated furnace, and the heating at the temperature from 1400 to 3000"F. is usually continued for a 7 period of minutes to hours, more often from 1 to 16 hours, in order to allow didusion of the nitrogen inv the mixture.

The furnace for the'heating operation is usually lined with graphite or, especially in the caseof hafnium, with a carbide of silicon or boron.

' weight percent of K ZnF in sodium chloride.

Patented ec. 5, 1%61 In the foregoing process the relative amounts of the nitrided metal powder and the powdered metal are usually in the ratio of 0.5 to 10 parts by weight of nitrided metal powder for each part by weight of the powdered metal.

The foregoing method, after cooling of the article, results in a non-brittle metal part of increased hardness throughout; in the case of titanium the hardness is usually in the range from to 800 Vickers hardness number. Such articles or parts can thereafter be further treated by nitriding in a nitrogen atmosphere to increase the surface hardness. In the case of titanium this surface hardness is usually in the range from 800 to 1200 Vickers hardness number. If it is desired to nitride the surface in accordance with this aspect of the invention the metal article can be left in the evacuated furnace in which it was treated at a temperature in the range from 2800- 3000 F. The temperature is lowered to the range from 1,000 to 2,000 E, usually 1600 to 1800 F, and a slow stream of purified nitrogen is introduced to the evacuated furnace. The usual treatment is for a period from 30 minutes to 16 hours.

As the nitrided powdered metal employed in the present process, the nitrogen content of the powder employed is usually in the range from 9.2 to 11.7 weight percent in the case of titanium, from 10 to 12.5 weight percent in the case of zirconium and from 9.7 to 10.5 weight percent in the case of hafnium.

The nitrogen employed in the step of nitriding the surface of the metal article molded from the powdered materials should be substantially free from hydrogen and water, since water decomposes to form hydrogen and oxygen. Any impurities such as oxygen and hydrocarbons should also be removed since both oxygen and carbon have a deleterious effect upon thejproperties of the nitrided metal, such as the impact rmistance. The presence of carbon results in surfaces formed which are rough, porous and loosely adherent.

The powdered metals can be prepared by suitable methods known in the prior art. Methods for preparing these metals are discussed, for instance, in the work, Rare Metals Handbook, edited by Clifford A. Hampel, Reinhold Publishing Corp., New York (1954). One method for preparing titanium powder is by the reaction of titanium tetrachloride with metallic magnesium, followed by washing the magnesium chloride from the finely-divided, dispersed, powdered, titanium metal with cold hydrochloric acid. in a somewhat preferred method for obtaining the titanium metal powder, the titanium metal prepared as in the foregoing method is purified by the so-called iodide method in which the powdered titanium is packedinto the periphery of a glass tube followed by evacuation of the system. A titanium filament is introduced into the unpacked portion of the glass tube into which has been introduced a small quantity of iodine. The titanium filamerit is electrically heated to 2372 F. and the entire tube is heated to about 302 F. This causes iodination of the crude titanium. When the titanium tetraiodide vapors impinge upon the titanium filament, the iodide is filament.

drawn into wires, or which upon being melted in a carbide clucible will form a compact metal rod which can be comminuted to a line powder suitable for use. in the process of the present invention. 7

Powderedzirconium can be formed by electrolyticprecipitation from a fused salt bath comprising 25 to 30 suiting powdered product is screened and further coinminuted to produce a fine powder.

Powdered hafnium canbe prepared in a similarrnan-p ner after concentrating a hafnium zirconium mixture by 3,dll,927

The re-;

any of the various known methods such as fractional precipitation, fractional decomposition, fractional distillation, solvent extraction, ion exchange, adsorption or reduction. The product can be purified by the tetraiodide method in a manner similario that utilized in the purification of titanium. The resulting crystal bar from the iodide precipitation is melted in an arc furnace under an argon atmosphere in a crucible made of graphite or a carbide of silica or boron. The ingot thus formed can be annealed and rolled into thin sheets about 0.0005 inch thick, shredded and ground to form a uniform powder.

The nitrided metals in powder form for use as starting materials according to the present invention can be prepared in any suitable manner. In one method they can be prepared by nitriding the corresponding powdered metal. Preferably, of course, the nitrided metals in powdcr form should not be prepared by ammonia nitriding or by any method which will introduce hydrogen or carbon or oxygen into the metal. In one suitable method the powdered metal is placed in a vacuum furnace and the atmosphere is reduced to a few microns of mercury pressure, at least below 50 microns Hg, and held for several hours, usually about 6 to 18 hours. During the evacuation the temperature is held at about 1000 F. The temperature is then raised to the range 1500 to 2000 F., preferably 1600 to 1800 F., and a stream of purified and dried nitrogen is introduced. The nitriding is continued at this temperature for a period of about 3 to 8 hours, generally about 6 hours, until the nitrogen content has reached the desired range, usually within the range as specified hereinbefore.

Vickers hardness number referred to herein is a test wherein an apparatus with a diamond point in the shape of a square-based pyramid, the sides of which make an angle of 136 at the apex, is utilized. This point is loaded with the desired weight, and the weights employed are somewhat outside the normal Vickers range of to 150 kilograms, 100 gram weights being utilized since it is necessary to determine the hardness gradient through a relatively thin section of the metal when testing surface hardness, rather than to obtain a strict Vickers hardness which penetrates too deeply in order to be completely descriptive of the outer surface, while in the standard Vickers hardness test utilizing the normal weights the average hardness of the outer, perhaps 0.02 inch of the specimen, is determined. In determining surfaces hardnesses, it is desired to obtain a gradient across hardened sections having a total thickness in the range from about 0.01 to 0.05 inch. In order to obtain an accurate measure of the diagonal of the square impression made by the Vickers point, a rapid determination of this dimension is made by recording calipers graduated in microns. The measurement is made within an interval of seconds after the impression is made. Vickers hardness number is calculated from the formula 1.8544L VHN where d is the length of the diagonal of the square impression made in the metal by the diamond point in millimeters, and L is the load in kilograms. The specimen is moved to within one millimeter of the diamond point and the loaded pyramid is lowered onto the specimen by means of a dashpot controlled cam mechanism which limits the duration of the application of the load to seconds. The load is then lifted and rotated away from the specimen and the photomicrograph is rotated into position to measure the diagonal. K

As a specific example of a metal part made in accordance with the invention, powdered titanium metal made by the iodide method and having a particle size in the range of 1 to 5 microns, is nitrided at a temperature of 1600 F. for a period of 6 hours. Ten grams of the resulting nitrided powder containing 11.5 weight percent nitrogen is mixed with 2 grams of the powdered metal. The mixture is placed in a mold suitable for preparing a strip having a thickness of 0.05 inch, a width of 1 inch and length of 3 /2 inches. The mold assembly is sealed so that the powder can be evacuated to remove any oceluded gases and vacuum is applied to evacuate to about 40 microns Hg absolute pressure, and then a pressure of tons per square inch is applied. The strip is removed from the mold and twostrips are cut from the central strip of the metal /2 inch Wide by 1% inches long. The two pieces thus prepared are placed in a silicon carbide lined vacuum furnace and heated to 1000 F. for about 1 hour, followed by heating at temperatures of 2850 to 2875 F. *for a period of 6 hours, followed by slowly cooling the furnace temperature to the range 1675 to 1700 F. and holding this temperature for a' period of 16 hours while introducing a slow stream of purified and dried nitrogen. The furnace is cooled down to about 1000 F, the furnace is evacuated and permitted to cool below 200 F. before removal of the pieces. Vickers hardness number on the strips prepared in this manner on various areas of the surface range from about 950 to 1100. The impact resistance of the strips is over 1.65 foot-- pounds.

As will be evident to those skilled in the art, various modifications of this invention can be made or followed in the light of the foregoing disclosure and discussion without departing from the spirit and scope of the disclosure or from the scope of the claims.

I claim:

1. A process for preparing a hard nitrided metal article which comprises intimately admixing a powdered metal selected from the group consisting of titanium, zirconium and hafnium with a nitride powder of said selected metal, molding the resulting intimate admixture to form a molded article from said admixture, and heating the resulting molded article at a temperature in the range from 1400-3000" F. in a substantially evacuated heating zone.

2. A process for preparing a hard nitrided metal article which comprises intimately admixing 0.1 to 2 parts by weight of a powdered metal selected from the group consisting of titanium, zirconium and hafnium with 1 part by weight of a nitride powder of said selected metal, molding the resulting intimate admixture to form a molded article from said admixture, and heating the resulting molded article at a temperature in the range from 1400- 3000 F. for a period from 15 minutes to 20 hours in a substantially evacuated heating zone.

3. A process for preparing a hard nitiided metal article which comprises intimately admixing 0.1 to 2 parts by Weight of a powdered metal selected from the group consisting of titanium, zirconium and hafnium with 1 part by weight of a nitride powder of said selected metal, molding the resulting intimate admixture to form a molded article from said admixture, and heating the resulting molded article at a temperature in the range from 2000- 3000 F. in a substantially evacuated heating zone.

4. Process of claim 1 wherein said metal is titanium.

5. Process of claim 1 wherein said article is surface ni-trided by contacting with nitrogen at a temperature from 1000 to 2000 F. 

1. A PROCESS FOR PREPARING A HARD NITRIDE METAL ARTICLE WHICH COMPRISES INTIMATELY ADMIXING A POWDERED METAL SELECTED FROM THE GROUP CONSISTING OF TITANIUM, ZIRCONIUM AND HAFNIUM WITH A NITRIDE POWDER OF SAID SELECTED METAL, MOLDING THE RESULTING INTIMATE ADMIXTURE TO FORM A MOLDED ARTICLE FROM SAID ADMIXTURE, AND HEATING THE RESULTING MOLDED ARTICLE AT A TEMPERATURE IN THE RANGE FROM 1400-3000* F. IN A SUBSTANTIALLY EVACUATED HEATING ZONE. 